Pluto’s Sputnik Planum is an ocean of slowly shifting nitrogen ice

Convection within the ice renews the surface, keeps it crater-free.

The stunning images returned from New Horizons' flyby of Pluto revealed a tremendous amount of information about the dwarf planet's features. That's been followed with the long, hard slog of trying to figure out how these features got there. One of the most striking things that needs an explanation is the apparent youth of Pluto's surface, as some areas appear to be crater-free, including the huge area called Sputnik Planum.

Now, researchers are offering an explanation for Sputnik Planum's apparent youth. Two papers in this week's edition of Nature indicate that radioactivity from Pluto's core would be sufficient to power convection of nitrogen ice. But the huge volume of ice involved creates another mystery, as it appears that almost all of Pluto's inventory of this element somehow ended up in Sputnik Planum.

Enlarge/ Sputnik Planum, the white plains to the left of center in this view.

Here on Earth, nitrogen is a gas making up the majority of our atmosphere. Those who have spent some time in a lab may be familiar with its liquid form, used for things that have to be cooled well below environmental temperatures. But on Pluto, it's typically cold enough—about 35K—that the majority of the dwarf planet's nitrogen is in solid form. This nitrogen ice has a couple of unusual properties. One is that it's much denser than water ice, which would allow the equivalent of icebergs to float on its surface. The other is that, since it's not held together by strong interactions among nitrogen molecules, it's relatively easy to deform.

Imaging of Sputnik Planum indicated it's largely composed of nitrogen ice, with some methane and carbon monoxide mixed in. But it also revealed some striking details: a surface honeycombed with irregular-shaped bulges, often tens of kilometers across and rising up to 50 meters above the plains. The borders between these polygons is lower elevation but is sometimes filled with small, jagged peaks—these are thought to be made of water ice that's floating on top of the nitrogen.

To figure out how these polygons formed, researchers focused on the possibility of convection within the nitrogen ice. Pluto does have a rocky core; based on the planet's density, it's probably about 900km across. If the core is formed from material typical of other bodies in the Solar System, it would contain some long-lived radioactive elements. The heat of their decay is minuscule: only three miliWatts for every square meter of surface. But it just might be enough to power convection.

Both teams of researchers set up models of Sputnik Planum with a small heat source at the base and filled it with nitrogen ice. In both cases, convection produced similar polygons and occurred fast enough to drive a turnover of the surface of Sputnik Planum in less than a million years. That nicely explains the lack of craters in this area. One of the two teams even suggests there might be transitions between two phases of the nitrogen ice, with the internal heat creating a less dense phase, and a more dense phase forming and sinking at the edges.

Both also calculate that, given the basin's size and likely depth, it holds all of the nitrogen that we'd expect Pluto to have accumulated when it formed. The two papers also agree that the reason for this is a bit of a mystery. "We do not at present understand why most of the N2 on Pluto is concentrated in what appears to be the basin of a large ancient impact crater," one team writes.

But they disagree on the nature of that basin. One team calculates that, if the objects in the polygon borders are really mounds of water ice, then for them to float, the nitrogen ice must be much deeper than we'd expect from an impact basin—at least five kilometers deep. The other paper suggests that it could be as little as three kilometers deep and still support the convection needed to reshape the surface.

In either case, the papers paint a vivid picture of a world with a single ocean, filled with nitrogen ice. That ocean is fed by "rivers"—nitrogen glaciers that drop from the mountains surrounding Sputnik Planum—that carry the chunks of water ice that we now see dotting its surface.